Allow trailing commas in function parameter lists

src/flag-definitions.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This file defines all of the flags.  It is separated into different section,
 // for Debug, Release, Logging and Profiling, etc.  To add a new flag, find the
 // correct section, and use one of the DEFINE_ macros, without a trailing ';'.
 //
 // This include does not have a guard, because it is a template-style include,
 // which can be included multiple times in different modes.  It expects to have
@@ skipping 192 matching lines @@
   V(harmony_function_sent, "harmony function.sent")                     \
   V(harmony_sharedarraybuffer, "harmony sharedarraybuffer")             \
   V(harmony_simd, "harmony simd")                                       \
   V(harmony_explicit_tailcalls, "harmony explicit tail calls")          \
   V(harmony_do_expressions, "harmony do-expressions")                   \
   V(harmony_restrictive_generators,                                     \
     "harmony restrictions on generator declarations")                   \
   V(harmony_regexp_named_captures, "harmony regexp named captures")     \
   V(harmony_regexp_property, "harmony unicode regexp property classes") \
   V(harmony_for_in, "harmony for-in syntax")                            \
-  V(harmony_async_await, "harmony async-await")
+  V(harmony_async_await, "harmony async-await")                         \
+  V(harmony_trailing_commas_in_parameters,                              \

[adamk, 2016/06/28 21:10:51]

How about "harmony_trailing_commas" as the flag name, for the sake of brevity?
That might help the code read a little better too.

+    "harmony trailing commas in function parameter lists")
 
 // Features that are complete (but still behind --harmony/es-staging flag).
 #define HARMONY_STAGED_BASE(V)                                               \
   V(harmony_regexp_lookbehind, "harmony regexp lookbehind")                  \
   V(harmony_tailcalls, "harmony tail calls")                                 \
   V(harmony_object_values_entries, "harmony Object.values / Object.entries") \
   V(harmony_object_own_property_descriptors,                                 \
     "harmony Object.getOwnPropertyDescriptors()")                            \
   V(harmony_string_padding, "harmony String-padding methods")
 
@@ skipping 969 matching lines @@
 #undef DEFINE_ALIAS_FLOAT
 #undef DEFINE_ALIAS_ARGS
 
 #undef FLAG_MODE_DECLARE
 #undef FLAG_MODE_DEFINE
 #undef FLAG_MODE_DEFINE_DEFAULTS
 #undef FLAG_MODE_META
 #undef FLAG_MODE_DEFINE_IMPLICATIONS
 
 #undef COMMA